High performance lubricants are used for a large number of diverse applications. The requirements of these lubricants are becoming more demanding due to a variety of factors, including miniaturization of electronic and mechanical devices, use of high temperature operating conditions, an increased expectation of product lifetimes, and an expanding range of operating and storage environments. In addition to providing acceptable lubrication to surfaces, lubricants also may need to offer thermal stability, chemical inertness, wear resistance, low volatility and/or corrosion resistance, depending on the particular application.
One application in which high performance lubricants are subject to ever increasing demands is hard disk drive magnetic recording systems. These systems use a collection of rigid disks, each coated with a thin layer of magnetic material and then with a protective layer, typically of amorphous carbon. The lubricant is applied to the surface of the disk over the protective layer. Due to the demand for higher density storage and faster data transfer rates, the lubricant film typically is only 1-2 nanometers (nm) thick. A disk is rotated at speeds greater than 15,000 revolutions per minute (rpm) during operation, which can cause non-uniform distribution of the lubricant or, in some cases, spin-off of a portion of the lubricant. In addition, these devices are expected to last for approximately 1010 revolutions, including an estimated 7,500 start-stop operations. Hard disk drives are expected to operate acceptably from 5° C. to 60° C. and from 8% to 80% relative humidity, and shipping and storage environments can range from −40° C. to 65° C. In order to avoid failure, the lubricant should not volatilize or experience hydrolytic or thermal instability during the life of the hard drive.
Conventional hard disk drive lubricants are typically perfluoropolyethers (PFPE's), such as the Fomblin® PFPE's available from Solvay Solexis, Inc. (Thorofare, N.J.). The performance and lifetime of PFPE lubricants can be limited by their thermal and chemical stability, as well as their static friction and adhesion properties. Local temperatures on the disk surface can reach 200° C. or greater during the recording process, a temperature at which PFPE's typically degrade and vaporize. Degradation of fluoroethers also can occur chemically at elevated temperatures by exposure to the Lewis acidity of the Al2O3/TiC material typically present on read-write heads, which record or read the information in the magnetic layer. The static friction coefficient of PFPE's can be prohibitively high as the lubricant film thickness increases, placing an upper limit on the amount of lubricant that can be present to protect the disk.
Aliphatic polyesters were studied as a possible improvement over PFPE lubricants. In a Master of Science thesis at University of Illinois in 1997 entitled “The Applications Of Advanced Materials To Four Unique Problems Involving Surface Interfaces”, by Brian Fabio, chapter 4 (pp. 61-89) disclosed a polyester formed from 2,2-diethyl-1,3-propanediol, 2-ethyl-2-methyl-1,3-propanediol, and 2,2,6,6-tetramethylheptanedioic acid; and a polyester formed from these monomers in addition to 2,2,5,5-tetramethylhexanedioic acid. Although these polymers are disclosed as having potential for general lubrication, the thesis specifically states that it could not be predicted whether the polymers would be useful for lubrication of hard disk drives until specific tests are carried out (pp. 75, 79, 85, 87-88). These tests include friction tests at normal loads of 2-4 grams and surface energy measurements performed in a controlled atmosphere.
A lubricant that has increased thermal and chemical stability would be beneficial. It would be desirable to provide more stable lubricants that could also have friction and wear properties that are comparable to or better than conventional lubricants.